Brazing is used for the materially integral connection of elements, in this case, inter alia, also for the connection of ceramic to metallic elements. One problem in relation to such connections, particularly in the field of gas turbine applications, is the poor wetting behavior of common brazing alloys on ceramic surfaces, such as are used in regions exposed to the hot combustion gases of turbines.
One possible solution to the problem, resulting from this, of poor materially integral connection after the brazing process is to metalize the ceramic surface prior to brazing. However, this entails major disadvantages in technical and economic terms.
A further possibility is to use so-called active brazing alloys. Active brazing alloys are often employed for connecting ceramic to metallic structural elements. They are distinguished by a certain content of elements with high oxygen affinity, thereby affording for the first time the preconditions for the wettability of the surface of the ceramic body. The alloys employed in this case usually contain, for example, Ti, Hf, Zr, and/or V as active elements. The term “active” therefore means, in this context, that, during the brazing process, these elements react with the ceramic surface to form a reaction layer (for example, Ti forms a layer of TiO2), this reaction layer being arranged between the ceramic element and the molten brazing alloy. This reaction layer acts as a connecting element between the ceramic and the brazing alloy.
Such commercial brazing alloys are typically based on Ag, Au, Cu, Ni or a mixture of the systems comprising Au/Ni and Ag/Cu, V or Ti normally being added in relatively small quantities as active element. These materials are generally applied to the ceramic brazed joint as foils or pastes or in powder form.
The problem with such brazing systems is their inherently low melting point Tmp, this greatly restricting the possibilities of use in the high-temperature range, that is to say, for example, in gas turbine applications. The melting points of the abovementioned brazing alloys are typically in the range of Tmp=780-960° C. for the Ag/Cu brazing alloys and Tmp=950-1050° C. for the Au/Ni brazing alloys. In such applications, the operating temperatures may enter the range of melting points or be even above these and therefore adversely influence the brazed joint. For gas turbine applications, a brazed connection has to withstand these high operating temperatures in the long term and always ensure a sufficient mechanical (materially integral) connection, this often not being the case with these brazing alloys.
The Cu- and Ni-containing brazing alloys are not resistant in the long term, in the combustion gas, to oxidation or corrosion caused by, for example, O2, H2O and SO2 constituents of the combustion gas. Active brazing alloys can be used as wire and foil material in the form of composition materials, for example as a eutectic Ag/Cu core with a Ti sheath or semifinished products laid one on the other and in each case composed of Ag or Zr. Furthermore, EP 0 038 584 A1 describes applying such materials on both sides to a brazing strip in the form of a sheet-like metal carrier (band) made from Cu or from a copper alloy.
There is also another group of active brazing alloys, to be precise those which are based on variable systems, such Pt—Cu—Ti (Tmp=1080-1770° C.) and Pd—Ni—Ti (Tmp=1080-1550° C.). This system has the disadvantage, however, of a very high melting point, this entailing problems with the metallic element to be connected.
To be precise, a high temperature during brazing entails undesirable variations in the microstructure particularly of superalloys (such as are used for components in gas turbine plants). These variations in microstructure often lead to mechanical weakening and fatigue. Moreover, such brazing alloys are generally less compatible with the customary Ni- and/or Co-based superalloys of the metallic components than are common Ni-based brazing alloys. This leads, for example with the aluminum of superalloys, to the formation of brittle intermetallic phases in the interdiffusion zone and consequently to a lower strength of the connection and, in general terms, to lower mechanical integrity of the hybrid system composed of the metallic element, ceramic element and brazing alloy lying between them.